is it possible to customize the CRM Listpart that is delivered by Microsoft. I would like to add a dropdown listbox to the presentation in CRM based on the documenttype in the library.
This webpart (if you are talking about the one for 4.0) is not supported from Microsoft anymore, and is only provided as-is.
The more common way to list data from Dynamics CRM in SharePoint would be to use BCS
Here is a video and some helpful links from MSDN blogs that will get you started with than in an instance!
Also this is a great resource about the matter.